The present invention relates in general to high power, high temperature semiconductor electronic switches, and, in particular, to silicon carbide gate turn-off thyristors.
The following publications discuss the problem of hole injection in silicon carbide (SiC) gate turn-off (GTO) thyristors:
1) P. B. Shah, K. A. Jones, A. K. Agarwal, S. Seshadri, xe2x80x9cIn-depth analysis of SiC GTO thyristor performance using numerical simulationsxe2x80x9d, Solid State Electronics, p. 353-358, vol. 44/2, 2000;
2) A. K. Agarwal, S. Seshadri, M. MacMillan, S. S. Mani, J. Cassady, P. Sanger, and P. Shah, xe2x80x9c4H-SiC p-n diodes and gate turn-off thyristors for high power, high temperature applicationsxe2x80x9d Solid State Electronics, p. 303-308, vol. 44/2, 2000; and
3) L. Cao, xe2x80x9c4H-SiC gate turn-off thyristor and merged p-i-n and schottky barrier diodes,xe2x80x9d Ph. D. Dissertation, Rutgers University, New Jersey, 1999.
A general discussion of GTO thyristors may be found in:
1) M. Azuma and M. Kurata, xe2x80x9cGTO Thyristorsxe2x80x9d, Proc. of the IEEE, vol. 76, p. 419, 1988; and
2) B. J. Baliga, Power semiconductor devices, PWS publishing company, New York, 1996.
Current crowding in GTO thyristors is discussed in:
1) R. Dutta, C. Tsay, A. Rothwarf, and R. Fischl, xe2x80x9cA physical and circuit level approach for modeling turn-off characteristics for GTO""sxe2x80x9d, IEEE Trans. on Power Elect., vol. 9, p. 560, 1994;
2) T. Yatsuo, S. Kimura, Y. Satou, xe2x80x9cDesign considerations for large-current GTO""sxe2x80x9d, IEEE Trans. Elect. Dev., p. 1196, vol. 36, 1989; and
3) T. K. Lee, Y. C. Liang, xe2x80x9cMinimization of current crowding during turn-off of power GTO devicesxe2x80x9d Power Electronics Specialists Conference, PESC ""94 Record, 25th Annual IEEE, p. 442, vol.1, 1994.
Additional information about SiC GTO thyristors appears in:
1) B. Li, L. Cao, and J. H. Zhao, xe2x80x9cHigh current density 800-V 4H-SiC gate turn-off thyristorsxe2x80x9d, IEEE Elect. Dev. Lett., vol. 20, p. 219, 1999;
2) R. R. Siergiej, R. C. Clarke, S. Sriram, A. K. Agarwal, R. J. Bojki, A. W. Morse, V. Balakrishna, M. F. MacMillan, A. A. Burke, Jr., and C. D. Brandt, xe2x80x9cAdvances in SiC materials and devices: an industrial point of viewxe2x80x9d, Mat. Sci. and Eng. B, vol. 61-62, p. 9, 1999;
3) K. Xie, J. H. Zhao, J. R. Flemish, T. Burke, W. R. Buchwald, G. Lorenzo, and H. Singh, xe2x80x9cA high current and high temperature 6H-SiC thyristor,xe2x80x9d IEEE Elect. Dev. Lett., vol. 17, p 142, 1996;
4) P. B. Shah and K. A. Jones, xe2x80x9cTwo-dimensional numerical investigation of the impact of material-parameter uncertainty on the steady-state performance of passivated 4H-SiC thyristorsxe2x80x9d, J. Appl. Phys., vol. 84, p. 4625, 1998;
5) J. B. Casady, A. K. Agarwal, S. Seshadri, R. R. Siergiej, L. B. Rowland, M. F. Macmillan, D. C. Sheridan, P. A. Sanger, and C. D. Brandt., xe2x80x9c4H SiC power devices for use in power electronic motor controlxe2x80x9d, Solid State electronics, vol. 42, p. 2165, 1998; and
6) A. K. Agarwal, J. B. Casady, L. B. Rowland, S. Seshadri, R. R. Siergiej, W. F. Valek, and C. D. Brandt, xe2x80x9c700 V Asymmetrical 4H-SiC Gate Turn-Off Thyristors (GTO)xe2x80x9d, IEEE Elect. Dev. Lett., vol. 18, p. 518, 1997.
Silicon gate turn-off (GTO) thyristors are quite well developed. Silicon GTO thyristors are used commonly in high power conditioning circuits, in high voltage DC systems, and traction circuits. Silicon carbide (SiC) GTO thyristors, on the other hand, are only recently coming of age because of the difficulty in producing good silicon carbide wafers and epi-layers. Silicon GTO thyristors appear to be inferior to SiC GTO thyristors because of their inability to operate at high temperatures (above xcx9c150xc2x0 C. junction temperatures). Further, silicon GTO thyristors tend to turn off slowly if designed to block very high voltages. Silicon carbide offers greater benefits than silicon for high power devices. These benefits include a higher thermal conductivity, a higher critical electric field at which breakdown occurs, and a high saturated carrier velocity.
With a higher breakdown field, thinner SiC devices can be developed that block a given amount of voltage, compared to silicon devices. The thinner SiC devices are faster switching devices because a smaller volume of charge carriers have to be removed during turn-off. Therefore the power handling capability of SiC GTO thyristors is much better than silicon GTO thyristors. SiC is also physically rugged and chemically inert.
In SiC GTO thyristors, it has been observed that if the gated base region is relatively high doped, the SiC GTO thyristor does not turn on at low current levels. This is because, in the anode, the acceptors being used in silicon carbide require a large amount of energy to be fully ionized so that very few are ionized at typical device operating temperatures. Therefore, there is a low injection of holes from the anode region into the center low doped regions of the SiC GTO thyristor resulting in poor conductivity modulation and poor on state characteristics. A thyristor that is unable to turn-on at low currents is also referred to as one that has a high holding current. On the other hand, a high doped gated base region is desirable because it minimizes current crowding that leads to device failure. As the capability to grow high quality SiC wafers and epi-layers improves, industry will focus more on larger area devices where current crowding will be an important issue.
The applications for SiC GTO thyristors include high voltage DC systems, traction circuits, motor control, power factor control, and other power conditioning circuits. These systems may be found in electric or hybrid electric tanks, electric helicopters, and other vehicles used by the military. Widespread application in these areas will occur if two issues are addressed. First, the voltage drop across the thyristor in its on state should be reduced. Second, the holding current should be as low as possible. A holding current of 1 A/cm2 has been recently stated as desirable. Also, to control large amounts of current as would be required in high end systems, rather than connecting many small devices in parallel it would be better to make several single large area devices. The material quality is expected to improve, but because of the large ionization energy of the acceptor dopant atoms that are currently used in silicon carbide, new techniques to improve the injection efficiency of holes from the anode region should be developed. The present invention, the modified anode gate turn-off (MA-GTO) thyristor, addresses these issues.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the following drawing.